Stone projectile motion problem

[FocusedWolf]

A stone is projected from ground level at 80 m/s. Its speed when it reaches its highest point is 60 m/s.

Find the angle of its projection. Answer in units of degrees.

Lost count how many times i tried to get this one right.

 

[Nylex]

Post your working, so we can show you where you've gone wrong.

 

[FocusedWolf]

OMG! i did it...it was my calculators fault! can't believe this.

here try this if you have a ti-89 (set mode to degrees)
(note the real values of the problem were 56m/s and 72m/s)

solve(56 = 72 * cos(x), x)

the answer it gives is weird

x = 360. * ((weird symbole)n8 + .108173447969) or x = 360. * ((weird symbole)n8 - .108173447969)

everything in my work made sense so i tried to do that manually:

56/72 = 72/72 * cos(x)
.77778 = cos(x)
cos-1(.77778) = x
x = 38.9


Now the weird thing...putting this in calculator backwards with the value i determiend for theta:

72 * cos(38.9) gets me 56! Freekin modern technology lol

o yea if anyone else wants to know how to do this problem, remember that in problems they neglect wind resistence the horizontal velocity is constant...so at the projectiles heighest point its velocity in the y direction is zero, so the velocity in the x direction is its total velocity...so now you have v0x...and they gave v0 so the formula v0x = v0 * cos(theta) will get you theta...i just figured this out now because i wanted to organize my messed up attempts for posting :P